This invention relates generally to multistage centrifugal pumps and more particularly to expansion compensators for multistage centrifugal pumps.
Double case, radially split barrel pumps, such as the pump shown in FIGS. 1 and 3, are used extensively in high pressure boiler feed service. The double cased design consists of a channel ring assembly 24, or bundle, which seals inter-stage pressure. The bundle 24 is fit into an outer pressure vessel, known as a barrel 3. Inside each channel ring 24 is a stationary diffuser 22 and a rotating impeller 15. Frequently in boiler feed applications, an intermediate stage pressure bleed-off line 35 is required. The bleed-off fluid is used for reheater attemporation spray which regulates reheater temperature. When an intermediate stage take-off 35 is required, intermediate stage pressure exists in the clearance between the channel ring assembly or bundle 24 and the barrel 3. Spiral wound gaskets 31 seated against machined steps in the barrel 3 seal the intermediate pressure from the suction and discharge pressure. In addition to creating a static seal between joints, gaskets 30 (discharge gasket), 31 also act as an expansion compensator. This permits the bundles 24 to expand within the barrel 3, allowing for differential thermal expansion and preventing distortion or misalignment of the internal components.
In existing compensator designs, shown in FIG. 3, erosion of the barrel base metal has been experienced around the suction gasket 32, primarily in turbine-driven pumps which are operated frequently at low speeds (the slow-roll condition). Here, the axial force generated by pressure is less that the minimum gasket seating load, resulting in an improper seal and washing in the barrel at the suction gasket (shown by arrow 34).
Another potential problem with existing compensator designs may occur when a pump is on stand-by. Spiral wound gaskets are crushed to a designed compression during pump operations. The gaskets are plastically deformed and do not retain the pre-stressed geometry. When the pump is stopped and placed on stand-by, the gaskets are unloaded. Due to the limited recovery characteristics of a plastically deformed spiral wound gasket, a gap 38 can exist between the bundle 24 and the barrel 3. This can cause erosion in the discharge head.
The foregoing illustrates limitations known to exist in present multistage centrifugal pump expansion compensators. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.